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Glossary term: Moons

Also known as natural satellite

Description: Moons are celestial bodies that orbit planets, dwarf planets or smaller objects such as asteroids. The Earth has one moon, called the Moon. Most other Solar System planets have moons, although Mercury and Venus do not. The dwarf planet Pluto has several moons as do a small number of other dwarf planets and asteroids. Moons are natural satellites, artificial satellites such as those used for communication or scientific purposes are not moons. Many moons formed in orbit around the planet, dwarf planet or other body that they orbit. It is though that the Moon formed orbiting the Earth. Many other (mostly smaller) moons are asteroids which were captured by the gravity of the object they orbit.

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Term and definition status: This term and its definition is still awaiting approval

The OAE Multilingual Glossary is a project of the IAU Office of Astronomy for Education (OAE) in collaboration with the IAU Office of Astronomy Outreach (OAO). The terms and definitions were chosen, written and reviewed by a collective effort from the OAE, the OAE Centers and Nodes, the OAE National Astronomy Education Coordinators (NAECs) and other volunteers. You can find a full list of credits here. All glossary terms and their definitions are released under a Creative Commons CC BY-4.0 license and should be credited to "IAU OAE".

Related Media

The planet Jupiter with the two of the four Galilean moons (visible as bright dots) orbiting it.

Jupiter's Rotation, by Vishal Sharma, India

Caption: Third place in the 2021 IAU OAE Astrophotography Contest, category Galilean moons: Jupiter’s Rotation, by Vishal Sharma, India. This time-lapse beautifully shows the rotation of Jupiter and the passage of two Galilean moons on the right side of the frame. Jupiter completes one rotation in just under 10 hours and we see as the Great Red Spot makes its way from left to right. The two moons travel a noticeable fraction of their orbit in this short time. This image was taken in 2020 in the North of India.
Credit: Vishal Sharma/IAU OAE
License: CC-BY-4.0 Creative Comments Attribution 4.0 International (CC BY 4.0) icons
The planet Jupiter, seen here as a bright disk, is orbited by the four Galilean moons, seen here as bright dots

Jupiter Moon's Movie2, by Nicolas Hurez, Paul-Antoine Matrangolo, and Carl Pennypacker, United States of America

Caption: Second place in the 2021 IAU OAE Astrophotography Contest, category Galilean moons. This sequence shows the orbit of the four Galilean moons around the planet Jupiter. Almost two entire orbits of the innermost moon, Io, can be seen, with the other moons (Europa and Ganymede, but in particular Callisto) being further away, orbiting noticeably slower. The images were obtained in 2018 with the Las Cumbres Global Observatory at different locations on Earth, allowing a continuous sequence of images over approximately half a week without gaps during the day. With clear skies and over the course of several nights, the motion of the Galilean moons can also be observed with binoculars (ideally steady your elbows on a surface).
Credit: Nicolas Hurez, Paul-Antoine Matrangolo and Carl Pennypacker/IAU OAE
License: CC-BY-4.0 Creative Comments Attribution 4.0 International (CC BY 4.0) icons
Jupiter with coloured horizontal bands of clouds. The shadow of the moon Io is seen as a dark circle in the top left

Jupiter, Io and its shadow, by Ralf Burkart, Germany

Caption: First place in the 2021 IAU OAE Astrophotography Contest, category Galilean moons. This time-lapse of Jupiter taken in 2017 from Germany beautifully illustrates the transit of one of the Galilean moons, Io, in front of Jupiter. As this is simply a moon casting a shadow on a planet it is equivalent to a lunar eclipse on Earth observed from further away. While the shadow of the moon is clearly visible from the beginning, it might be difficult to spot the moon itself against the background of the beautiful atmospheric bands of Jupiter the first time the video is seen. Watching it repeatedly allows appreciating the rapid motion and rotation in this fantastic observation.
Credit: Ralf Burkart/IAU OAE
License: CC-BY-4.0 Creative Comments Attribution 4.0 International (CC BY 4.0) icons
The nearly full Moon with craters, light highlands and dark plains

Full moon

Caption: The image shows the nearly full Moon observed with a small telescope and a DSLR camera.
Credit: Luc Viatour credit link
License: CC-BY-SA-3.0 Creative Comments Attribution-ShareAlike 3.0 Unported icons
Io is roughly spherical. Its surface mostly consists of yellowish sulphuric compounds and rather small darker volcanos.

Io

Caption: NASA's Galileo spacecraft acquired its highest resolution images of Jupiter's moon Io on 3 July 1999. Io is one of the four Jovian moons discovered by Galileo Galilei in 1610. Io's colours are witness to its extensive volcanic activity as they stem from sulphuric compounds. Tidal forces from Jupiter and the neighbouring moons are the cause for Io's volcanism.
Credit: NASA/JPL/University of Arizona credit link
License: PD Public Domain icons
The asteroid Ida is grey and shaped like a potato with lots of shallow craters. It's moon Dactyl is 40 times smaller

Ida and Dactyl

Caption: This picture of the asteroid Ida with its satellite Dactyl was taken by the Galileo space probe in August 1993 from a range of 10,870 kilometers (6,755 miles). Ida is a heavily cratered, irregularly shaped asteroid in the main asteroid belt between Mars and Jupiter. This image is proof that asteroids like the 56 km large Ida can possess their own moons.
Credit: NASA/JPL credit link
License: PD Public Domain icons
Europa is round with large patches of brown to white colours, covered by numerous crevices, randomly oriented on the surface

Europa

Caption: This image taken by NASA's Galileo space probe in September 1996 shows Jupiter's ice-covered satellite, Europa, in approximate natural color. Long, dark lines are fractures in the crust, some of which are more than 3,000 kilometers (1,850 miles) long. The bright feature containing a central dark spot in the lower third of the image is a young impact crater some 50 kilometers (31 miles) in diameter. Europa is about 3,160 kilometers (1,950 miles) in diameter, or about the size of Earth's moon.
Credit: NASA/JPL/DLR
License: PD Public Domain icons
Saturn's moon Titan, its atmosphere of dense clouds leads to an almost uniform orange-yellowish appearance

Titan in natural colours

Caption: This image taken by NASA's Cassini spacecraft in January 2012 shows Saturn's moon Titan in near natural colours. The orange-yellowsh colour stems from a layer of partially charged hyodrocarbon compounds. In the visible light, Titan's surface remains largely hidden below the opaque atmosphere.
Credit: NASA/JPL-Caltech/Space Science Institute credit link
License: PD Public Domain icons
Titan is round and this false colour image shows its surface as green with a large H-shaped dark patch

Titan with surface features

Caption: This composite image taken by NASA's Cassini spacecraft in November 2015 visualises infrared radiation from Saturn's moon Titan. Infrared light can penetrate its hazy atmosphere and reveal some of its surface features. During its decent to the surface, the piggy-backed ESA lander Huygens discovered lakes made of liquid hydrocarbons.
Credit: NASA/JPL/University of Arizona/University of Idaho credit link
License: PD Public Domain icons
A partially illuminated Enceladus. The smooth surface is covered by many  straight and curved crevices

Enceladus

Caption: On 9 Oct. 2008, just after coming within 25 kilometers (15.6 miles) of the surface of Enceladus, NASA's Cassini probe captured this stunning mosaic of Saturn's 500 km large ice-moon. Craters and cratered terrains are rare in this view of the southern region of the moon's Saturn-facing hemisphere. Instead, the surface is replete with fractures, folds, and ridges — all hallmarks of remarkable tectonic activity for a relatively small world. In this enhanced-color view, regions that appear blue-green are thought to be coated with larger grains than those that appear white or gray. Similar to Europa, Enceladus has a subsurface ocean of liquid water spanning the entire moon. A false colour image of a partially illuminated Enceladus. The white surface is covered by numerous straight and curved crevices at random orientations
Credit: NASA/JPL/Space Science Institute credit link
License: PD Public Domain icons
Pan is grey with a strange, shape almost like a ravioli with a few small craters and ridges

Pan

Caption: NASA's Cassini spacecraft captured this image of Saturn's moon Pan in March 2017. Pan is a so-called shepherd moon that orbits Saturn within the Encke gap of Saturn's rings. The striking thin brim stems from material Pan collects during its trajectory through the rings. Pan has a mean diameter of alomst 30 km.
Credit: NASA/JPL-Caltech/Space Science Institute credit link
License: PD Public Domain icons